This invention relates to a knock sensor for a spark ignited, internal combustion engine and particularly to such a sensor comprising a multiply tuned vibrator combined with a piezoelectric transducer.
It has been taught in the prior art that the phenomenon of knock in a spark ignited, internal combustion engine is associated with vibrations of engine components at specific frequencies but that there is sometimes a great deal of noise at frequencies close to the specific frequencies. In addition, the specific frequencies often vary slightly from one combustion chamber to another or with changing engine operating conditions. Therefore, it is desirable that the system have high frequency selectivity but with a passband somewhat wider than that available from a single tuned element.
The prior art includes interactive sensors, such as that shown in U.S. Pat. No. 4,254,354 to John E. Keem, issued Mar. 3, 1981, in which the resonant frequency is a function of the interaction between the sensor vibrating mass and the mass of the component on which it is mounted. The prior art also shows self resonant sensors, in which the resonant frequency is determined by the vibrating mass of the sensor only. Such sensors may use multiple separate vibrating elements having resonant frequencies slightly spaced from each other, as shown in U.S. Pat. No. 4,373,378 to Fujishiro et al, issued Feb. 15, 1983, or a single disk placed off-center, as in U.S. Pat. No. 4,371,804 to Peng et al, issued Feb. 1, 1983. In either case, however, special care must be taken in manufacturing for the correct placement of the vibrating element to ensure the desired resonant frequencies.